‘87 K75S Failed compression test - burned valve

[fuego]

Hi !

Upon the restoration of the 1987 K75S, it failed the compression test on the cylinder #2. puzzling is the fact that the engine was idling very smooth, plenty of power ( goes at 110 mph no problem), no vibration at any speed. The bike starts on first quarter turn. No smell in the exhaust, no oil consommation, no fumes.

Really puzzling...

But on cylinder 2 the compression was 40 psi, then 0 psi on the second go around. The engine was at operating temperature, throttle wide open. Cylinder 1 and 3 were 125 psi. The bike has 83 000 miles.

So I did this : removed the valve cover, made very sure with cam position that both valves (intake and exhaust) were closed, then push compressed air in the cylinder. Sure enough, there was a major leak in the exhaust port...

Is that a burned valve ? And what do I do with that, buy a valve, valve guides, valve seals and get the head rebuild ? Or should I just buy a known good used head. And if this, how do I know it's good if off the bike ?

Thanks for any help,

Ignace Speybrouck,

1987 K75S
1989 K75
Both undergoing a frame off restoration

1976 MGB (backdated to look like a 1969...)
1988 Mercedes 300E
1998 Mercedes E320

 

[98lee]

Are the valves properly adjusted? That would be the first thing to check. You want .012” clearance on the exhausts and .006-.008” on the intakes cold.

 

[fuego]

Are the valves properly adjusted? That would be the first thing to check. You want .012” clearance on the exhausts and .006-.008” on the intakes cold.

Hi Lee,

Yes, forgot to mention all the valve clearances are whithin specs. Actually to do the check,I could pass the feeler gage between the cam and the valve.

It's a good point though.

Ignace

 

[k1200lt00]

It's a slim change but they might be a piece of carbon stuck on the valve face or seat. You could pull the exhaust of and you might be able to see what the problem is before you remove the head.

 

[dravnx]

Shove a borescope in there and have a look at the exhaust valve. Lots of low cost borescopes that use your smartphone.

 

[fuego]

Excellent idea !

I will shove a borescope in there before removing the head !

I'll keep you posted, thanks for the input !

Ignace

 

[fuego]

Excellent idea !

I will shove a borescope in there before removing the head !

I'll keep you posted, thanks for the input !

Ignace

Well,

Nothing special to see in there, the 6 valves look all similar. I guess,the head will come off to have a look inside.

I will post pictures once removed.

Ignace

 

[k1200lt00]

Once you get the exhaust off shine a light in the exhaust port and then look in the cylinder and see what it looks like.

 

[fuego]

Thanks,

When I get smart advise like this I always wonder why my little brain can't come with !

I will do that tonight, I certainly gess that with a closed valve... no light !

I'll keep you posted.

Just wondering what to do if the valve is actually burned ?

Ignace

 

[Omega Man]

I'm curious on the borescope that was used? The inexpensive units don't really have the resolution to see what you need- from my experience.
FWIW, in the old days, carbon buildup was was removed by slowly pouring water down the carburetor while the engine was running about half throttle.
Not sure how this would work with fuel injection.
OM

 

[k1200lt00]

Water works good, but if it's carbon you can use Sea Foam and spray it in the plug hole and though the exhaust port on to the valve and let it sit for a while. It should losing up the carbon.

 

[pglaves]

Water works good, but if it's carbon you can use Sea Foam and spray it in the plug hole and though the exhaust port on to the valve and let it sit for a while. It should losing up the carbon.

True. But if carbon is holding a valve open it didn't originate there. It would be a chunk that broke loose and got trapped there.

This happened to both cylinders on Voni's R1100RS while riding in a very heavy rain storm. Water entered the forward facing air snorkel, soaked the air filter, and entered the cylinders while she was riding at speed down the interstate. When she took an off-ramp and stopped the bike dropped back to idle and didn't eject all the carbon past the valves and died.

I had to remove the heads and manually clean the valves and seats to get rid of the last few bits of carbon. The combustion chamber and piston tops looked almost new after this cleansing bath.

 

[fuego]

True. But if carbon is holding a valve open it didn't originate there. It would be a chunk that broke loose and got trapped there.

This happened to both cylinders on Voni's R1100RS while riding in a very heavy rain storm. Water entered the forward facing air snorkel, soaked the air filter, and entered the cylinders while she was riding at speed down the interstate. When she took an off-ramp and stopped the bike dropped back to idle and didn't eject all the carbon past the valves and died.

I had to remove the heads and manually clean the valves and seats to get rid of the last few bits of carbon. The combustion chamber and piston tops looked almost new after this cleansing bath.

OMEGA

The borescope I used is from RIGID. I work in a school board and I borrowed it. Very hi resolution !


As for the head, I guess it will come off for inspection. Interresting the water thing, but my engine isn't working yet !

Ignace

 

[dravnx]

Another trick is to pour kerosene in to the valve stem and then, using your borescope, see if it leaks past the valve.

 

[fuego]

Just wondering what to do if the valve is effectively not tight.

Does this mean the valve is burned ?

I need a new valve .

And finally are the valves of all the k75 from 1986 on the same ?

Ignace

 

[98lee]

All K75 exhaust valves are the same.

 

[fuego]

Thanks Lee,

The head is out, the verdict has fallen, burnef valve !

Why is that ? The bike rode so good !

Anyway, look at the piçtures, it's clear !

The bores look impeccable though.

What's next ?

Ignace

 

[Omega Man]

Nice job on this.

Around here it seems most of the machine shops (owners) have aged out. Every now and then MAX BMW sends out an email that happens to show their machine shop equipment in the background…..looks really well equipped.
Maybe you have someone near you that has similar equipment and skills?
OM

 

[98lee]

Thanks Lee,

The head is out, the verdict has fallen, burnef valve !

Why is that ?

Ignace

Someone neglected to stay on top of the valve adjustment!

 

[taran1900]

Someone neglected to stay on top of the valve adjustment!

I think so too. I have never heard of a burnt valve on any early K-bike, especially the K75. If you do proper maintenance on them those motors will easily go over 500 hundred thousand miles without having to to do anything major. Those motors were overbuilt and thus, under-stressed.